One of the characteristic features of human interaction is variety of expression. For example, even when two people interact repeatedly in a similar manner, such as greeting one another, many different expressions may be used despite the fact that a simple “hello” would suffice in almost every instance. Instead, human beings in interaction are likely to substitute “good morning”, “good evening”, “hi”, “yo”, or a non-verbal expression, such as a nod, for “hello”, depending on the context and the circumstances surrounding the interaction. In order for a non-human social agent, such as one embodied in an animated character or robot for example, to engage in an extended interaction with a user, it is desirable that the non-human social agent also be capable of varying its form of expression in a seemingly natural way.